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As you probably know, Britney Spears shaved her head this past week. Observers speculate that one reason she may have done so is to attempt to avoid a hair strand drug test in her ongoing custody case with her estranged husband, Kevin Federline.

I mention this not because I plan to start covering celebrity breakdowns and divorces on this blog, but rather because the subject of drug tests comes up quite frequently in Family Court cases. Slate.com published an article which serves as a good introduction to the subject of hair strand drug tests. For instance, consider the following facts from this article:

Drug testing is becoming increasingly common in custody battles as a way to prove a parent unfit or irresponsible.

A bundle of hair about the thickness of a pencil can tell chemists what specific drugs someone has used and provide a rough timeline of when she used them.

Narcotics like cocaine, meth, ecstasy, and PCP introduce toxins to the bloodstream that are then incorporated into each hair as it forms in the follicle.

Urine and blood only retain evidence of these toxins for a week or so, but hair can hold on to them indefinitely.

Head hair grows about half an inch every month, so a woman with shoulder-length hair carries around a two-year record of her drug use.

Drug tests can be done on any body hair—armpit, leg, back, pubic—but head hair is preferred because of its length and relative cleanliness. Although pubic hair may be long enough to provide a few months' worth of information, it is less reliable because of its likely contamination from sweat and urine.

Hair from any part of the body might become unreliable if it came into external contact with drugs—for example, if someone spent time in a room filled with crack cocaine smoke. Test results can also be affected by hair color or texture. Studies have shown that a dark strand will take up drug residues more readily than a light one—a peculiarity that has prompted some critics, including the American Civil Liberties Union, to deem the practice racist.

Hair testing for illicit drugs started in the late 1970s, but the practice did not catch on commercially until about 15 years ago, when improved technology allowed for more-accurate results.

When administered correctly, hair tests are about as accurate as urinalysis. However, there are still no federal standards for hair testing.